1. Field of the Invention
The invention relates to superconducting magnets. More particularly, a balanced quench protection circuit is provided to protect its superconductive assemblage from damage during a quench.
2. The Prior Art
As is well known, a magnet coil wound of superconductive material can be made superconducting by placing it in an extremely cold environment. For example, a coil may be made superconducting by enclosing it in a cryostat or pressure vessel containing a cryogen. The extreme cold enables the superconducting wires to be operated in the superconducting state. In this state, the resistance of the wires is practically zero. To introduce a current flow through the coils, a power source is initially connected to the coils for a short time period. In the superconducting state, the current will continue to flow through the coils, thereby maintaining a strong magnetic field. In other words, because superconductive windings offer no resistance to electrical current flow at low temperatures, the superconducting magnet is persistent. The electric current that flows through the magnet is maintained within the magnet and does not decay noticeably with time. Superconducting magnets find wide application in the field of magnetic resonance imaging (xe2x80x9cMRIxe2x80x9d).
In a typical MRI magnet, the main superconducting magnet coils are enclosed in a cylindrically shaped cryogen pressure vessel. The cryogen vessel is contained within an evacuated vessel and formed with an imaging bore in the center. The main magnet coils develop a strong magnetic field in the imaging volume of the axial bore.
A common cryogen is liquid helium. During superconducting operation, liquid helium boils to form helium gas. The gas is either recondensed for recycling and reuse or is vented to the atmosphere.
A major concern in such apparatus is the discontinuance or quenching of superconducting operation. Quenching can produce undesirable and possibly damaging high temperatures and voltages within the magnet. During a quench event, the current in the persistent superconducting coils decays rapidly. The rapid decay is as a result of resistive zone(s) developed in the coils for example due to thermal disturbance. Quenching may occur from an energy disturbance, such as from a magnet coil frictional movement. The energy disturbance heats a section of superconducting wire, raising its temperature above the critical temperature of superconducting operation. The heated section of wire becomes normal with some electrical resistance. The resulting I2R Joule heating further raises the temperature of the section of wire increasing the size of the normal section. An irreversible action called quench then occurs. During a quench, the electromagnetic energy of the magnet is quickly dumped or converted into thermal energy through the increased Joule heating.
For MRI application, a homogeneous magnetic field is required in the imaging volume. To provide the required homogeneity, the magnet coil is divided into a plurality of sub-coils. The sub-coils are spaced along and around the axis of the superconducting magnet such that they are not thermally connected. As a result, when only one of the superconducting coils quenches, the entire energy of the strong magnetic field may be dumped into the quenching coil. A hot spot and possible damage is caused unless a suitable quench system is provided. Quench protection can be accomplished by quickly quenching the other coils or sending part of the energy to dump resistors. Damage from rapid rise in temperature and voltage, or from quick electro-magnetic force change in the magnet, is thereby prevented.
A number of quench systems for protection of superconducting magnets are known. For example, U.S. Pat. No. 6,147,844 to Huang et al, U.S. Pat. No. 5,739,997 to Gross, and U.S. Pat. Nos. 5,650,903 and 5,731,939 to Gross et al relate to quench protection circuits for superconducting magnets.
An actively shielded MRI magnet consists of main coils and bucking coils. These coils produce a homogeneous field in the image volume and reduce fringe fields. Most of the superconducting MRI magnets are made of coils that are symmetric with respect to a symmetry mid-plane. During a quench, the current in different coils may not decay at exactly the same rate. A net differential force acting on the coils and coil supporting structure could thereby be induced. The net differential force in the right and left halves of the magnets and/or between the main coil and the bucking coil structures results in an unbalanced quench. An unbalanced quench could potentially damage coil supporting structures, depending on the severity of the unbalance. Therefore, an unbalanced quench is undesirable for supporting structures without a large built-in safety margin in the structural design. A large safety margin can increase cost and/or space occupation.
To solve the above problems, a superconducting magnet electrical circuit is provided. The circuit results in protection of a superconductive magnet through a balanced quench. A superconducting coil assemblage is provided including a plurality of spatially separated main and secondary magnet coil portions. The main magnet coil portions are connected in series to form at least one main coil series circuit. The secondary magnet coil portions are likewise connected in series to form at least one secondary coil series circuit. At least one temperature limiting circuit with a plurality of quench heaters or quench resistors is also provided. The temperature limiting circuit is connected in parallel with the superconducting coil assemblage. When a quench heater circuit is used, the circuit may include a plurality of quench heaters connected in parallel with each other. When a quench resistor circuit is used, the circuit may include a plurality of quench resistors connected in series or in parallel with each other.
Thus, the secondary magnet coil portions, for example, two bucking coils, are grouped together in a sub-circuit. The main magnet coils are likewise grouped together in a separate sub-circuit. In addition, a superconductive switch is coupled with the superconducting assemblage. When a quench occurs, the current flow through the coil (main or bucking) that initiated the quench will be the same as the symmetric (main or bucking) coil with respect to the mid-plane because they are connected in the same sub-circuit. Though currents in different sub-circuits may be different, the symmetry or current balance is preserved in terms of the current in the two half magnets. Therefore the force, acting on each half structure and/or the main and bucking structures, will be minimized, resulting in a balanced quench.
In one aspect of the invention, at least two quench heater or quench resistor circuits are connected in parallel with the superconducting coil assemblage. In another aspect of the invention, at least one quench heater circuit and at least one quench resistor circuit are connected in parallel with the superconducting coil assemblage. The quench heater circuit includes a plurality of generally identical quench heaters connected in parallel. The quench resistor circuit includes a plurality of generally identical quench resistors connected in series or in parallel with each other.
In a further aspect, at least one quench heater is positioned in thermal contact with the main magnet coil portions. Likewise, at least one quench heater is positioned in thermal contact with the secondary magnet coil portions.
In another aspect, the magnet coils of the superconducting coil assembly are essentially completely in contact with a fluid cryogen and at least one temperature limiting circuit is connected in parallel with the superconducting coil assemblage.